Hearing aids are generally operated using a voltage source in the form of a standard battery or a rechargeable battery. These are generally discharged even after just a few days of operation and must be replaced or recharged. It is thus desirable for the hearing aid wearer to be informed in good time that the voltage source is running down. By way of example, U.S. Pat. No. 6,320,969 discloses a hearing aid which monitors the charge status of the voltage source and warns the user that the voltage source is running down by emitting an acoustic signal.